1. Field of the Invention
This invention relates to a waveguide type electro-optical element comprising an optical waveguide formed on a substrate possessing an electro-optical effect, and at least a pair of electrodes formed on the optical waveguide. The waveguide type electro-optical element is designed to effect modulation and switching of guided light by application of a voltage to the electrodes.
2. Description of the Prior Art
A waveguide type electro-optical element has already been known such as disclosed, for example, in Unexamined Japanese Patent Publn. No. HEI-2(1990)-931 incorporated herein by reference. The electro-optical element comprises a thin film optical waveguide formed on a substrate possessing an electro-optical effect, a grid-shaped electrode (hereinafter referred to as an EOG electrode) being formed on the substrate and constituting electro-optical gratings in the optical waveguide, and a drive circuit for applying a voltage to this EOG electrode. Guided light, traveling through the optical waveguide, is selectively diffracted in accordance with a state of the application of voltage to the EOG electrode.
The use of such a waveguide type electro-optical element enables the modulation of light to be used in accordance with the existence or absence of diffraction or the degree of diffraction when either diffracted light or non-diffracted light (zeroth order light) is used as the light to be used. The use of the waveguide type electro-optical element also makes it possible to construct an optical switch which switches an optical path in accordance with the presence or absence of diffraction.
Another waveguide type electro-optical element is also disclosed, for example, in the Japanese Journal of Applied Physics, Vol. 20, No. 4 April, 1981, pp. 733 to 737, which is incorporated herein by reference. In this waveguide type electro-optical element, two channel optical waveguides which constitute a directional coupler are formed on a substrate possessing an electro-optical effect, and a planer electrode is provided on each channel optical waveguide. Guided light traveling through one of the channel optical waveguides is selectively switched to the other channel optical waveguide in accordance with application of a voltage to the electrode.
The use of such a waveguide type electro-optical element enables the modulation of light, being emitted from the other channel optical waveguide, in accordance with the state of the application of a voltage to the electrode, or makes it possible to constitute an optical switch which switches an optical path of the guided light.
In the waveguide type electro-optical element having such a construction, an optical buffer layer is often interposed between the electrodes and the substrate to prevent the scattering or absorption of light caused by the electrode. Conventionally, this buffer layer is made Of SiO.sub.2 or Al.sub.2 O.sub.3.
In a waveguide type electro-optical element comprising a proton-exchanged optical waveguide used as an optical waveguide and the above-mentioned buffer layer interposed between a substrate and electrodes, it is acknowledged that a sharp change in the temperature of the electro-optical element may cause guided light to be diffracted even though a voltage is not applied to the electrodes.
In this way, if the diffraction of the guided light occurs irrespective of the application of a voltage to the electrodes, it will be impossible to control the modulation of light or the switching of an optical path of guided light. Therefore, it will be difficult to construct an optical modulator and an optical switch which can be put into practical use by utilization of this waveguide type electro-optical element.
This invention is made in view of the foregoing drawbacks in the prior art, and the object of the present invention is to provide a waveguide type electro-optical element capable of preventing guided light from being diffracted if the temperature of the electro-optical element is sharply changed when no voltage is applied to the electrodes.